Random access memories, usually called RAMs and often direct access memories, are extensively used in virtually all digital computing equipment for the temporary storage of binary information which may be rapidly retrieved in a random search without first searching through a large volume of irrelevant data. There are several types of RAMs, the most popular being the bipolar memory because it is non-destructive and because of its manufacturing simplicity, low cost, high reliability, and low current demand. A bipolar RAM may be described as a matrix of a large quantity of individual memory cells with vertical columns of cells interconnected by bit lines, and horizontal rows interconnected by top and bottom pairs of word lines. Each cell is, in its simplest form, a bistable flip-flop consisting of two cross-coupled inverter circuits each usually employing a pair of transistors or a dual emitter transistor with one emitter of each inverter connected to the bottom word line that is connected to all cells in that horizontal row, and a second emitter coupled to one of two bit lines coupled to all cells in that vertical column. When one transistor of the flip-flop pair is enabled, it represents a binary "1", while the disabled transistor of the pair would be read as a binary "0". One of the bit lines in each column is connected to output sensing circuitry and also to a write amplifier. A selected cell is normally read by selecting and raising the voltage on the top word line to provide V.sub.cc to the cell and by providing a current source to the bottom word line. The voltage on the bit lines is maintained constant and the sense amplifier coupled to the bit line will sense the presence or absence of current in the bit line because of the ON or OFF state of the cell transistor connected to that bit line.
Writing into a particular cell is accomplished by raising the top word line of the selected cell and lowering the voltage on one bit line to enable the transistor connected to that bit line. Thus, to write into a particular selected cell a word line selecting circuit, such as a word line decoder, will raise the voltage on the top word line to thereby enable all the cells in that horizontal row and thereby permit each of the sensing circuits to detect the state of each cell in that selected horizontal row. Writing is accomplished by similarly raising the voltage on the word line in a selected horizontal row, but dropping of voltage on a selected bit line to thereby write into the cell at the junction of the selected bit line and word line. As long as a low standby current is provided to each cell, the RAM is said to be non-destructive so that all binary data entered into the memory will remain intact until rewritten.
It is often desired to reset a RAM so that all cells are reset to the same binary number. There are two presently accepted circuits for accomplishing this reset process. One method involves the construction of an unsymmetrical cell at each cell location. Presetting a matrix of unsymmetrical cells involves raising the word line voltage and dropping the voltages on both bit lines by an equal amount and, because of the lack of symmetry of each of the cells, one transistor of the flip-flop circuit will conduct heavier than the other transistor so that each cell will be set to this heavy conducting side. In general, this unsymmetrical cell method is unreliable because of its sensitivity to manufacturing process variations. A second method currently in use involves the use of an extra or third emitter on one or both sides of the cell flip-flop, thereby enlarging the size of the cell and its associated deposited conductors.
These problems are overcome by the present invention which employs simple small cell that is not process dependent and one which has the advantage of permitting the presetting of the RAM in either state.